spherical speaker enclosure

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I remember reading somewhere that spherical enclosures are ideal
shape..is that correct? debatable?
Incidentally, I live in India where we get *Very Cheap* earthen pots,
called "matkas" which are near perfect spheres with 'mouths emerging
from protruding necks', used for storing water, and they keep the water
inside cool (as they are porous I guess).

I wish to build a speaker enclosure using them, as they are ready made
in various sizes, have an artistic appeal and if the sound reproduction
dose not suffer, It's a very attractive proposition!
These pots or "matkas" have walls which are usually 3-4 Milimeter
thick ( I can get them made thicker).

I did some tries by mounting 8 inch drivers on the "mouth", did some
stuffing inside using polyfill, but the observed sound reproduction did
not seem too "right" to me- I thought it was resonating too much (like
booming) at low freqs. and had a tinny feel to the sound..

Please have a look at the setup here:
http://www.geocities.com/madforeagles/DSC01343.JPG
http://www.geocities.com/madforeagles/DSC01344.JPG


My queries are, should I line the insides with some material to make it
air tight? how thick should I get them made? should I use the same
volume calculations for 'normal' enclosures for a driver? should I
stuff it with Polyfill?

Thanks!
Suraj

 

[jutek]

Please have a look at the setup here:
http://www.geocities.com/madforeagles/DSC01343.JPG
http://www.geocities.com/madforeagles/DSC01344.JPG


My queries are, should I line the insides with some material to make it
air tight? how thick should I get them made? should I use the same
volume calculations for 'normal' enclosures for a driver? should I
stuff it with Polyfill?

hello

try to upload jpg's using www.imageshack.us, these links don't work now.

spherical enclosure causes we don't need a filler, but you can try.

i think 4mm is not enough. i would use 1.5-2.5cm, but i'm not sure it's
the right material.

regards

 

[Bob Masta]

I remember reading somewhere that spherical enclosures are ideal
shape..is that correct? debatable?

That would be debateable. I suspect you may be remembering that
a spherical *source* is best for omnidirectional sound radiation,
but there is no easy way to get that with an enclosure. Some
folks have tried covering a spherical enclosure with many small
drivers. The problem there is that, as with any multi-driver system,
you have interactions between the driver coverage patterns.
A driver's output pattern is dependent upon frequency, especially
toward the top of its range. So if you try to position the drivers
so that their fields just meet for perfect coverage at one frequency
range, then at other ranges the fields will have overlaps or gaps.
That gives ragged frequency response due to phase cancellation, etc.

Incidentally, I live in India where we get *Very Cheap* earthen pots,
called "matkas" which are near perfect spheres with 'mouths emerging
from protruding necks', used for storing water, and they keep the water
inside cool (as they are porous I guess).

I wish to build a speaker enclosure using them, as they are ready made
in various sizes, have an artistic appeal and if the sound reproduction
dose not suffer, It's a very attractive proposition!
These pots or "matkas" have walls which are usually 3-4 Milimeter
thick ( I can get them made thicker).

I did some tries by mounting 8 inch drivers on the "mouth", did some
stuffing inside using polyfill, but the observed sound reproduction did
not seem too "right" to me- I thought it was resonating too much (like
booming) at low freqs. and had a tinny feel to the sound..

Please have a look at the setup here:
http://www.geocities.com/madforeagles/DSC01343.JPG
http://www.geocities.com/madforeagles/DSC01344.JPG


My queries are, should I line the insides with some material to make it
air tight? how thick should I get them made? should I use the same
volume calculations for 'normal' enclosures for a driver? should I
stuff it with Polyfill?

The pot "enclosure" you describe would be a classic Helmholtz
resonator... the exact opposite of what you want for sound
REproduction, though it might be a great musical instrument on
its own. The polyfill will help you somewhat, but I suspect that
the bigger problem is that the walls themselves are resonating
like a big bell. If you are determined to follow through with
this (for the sake of art, say) then you might want to try
coating the pot walls (inside or out) with something dense and
viscous, like pitch. As another post notes, these walls are pretty
thin to start with, which is making the problem worse.

Best regards,


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator

 

[Bob Eld]

I remember reading somewhere that spherical enclosures are ideal
shape..is that correct? debatable?
Incidentally, I live in India where we get *Very Cheap* earthen pots,
called "matkas" which are near perfect spheres with 'mouths emerging
from protruding necks', used for storing water, and they keep the water
inside cool (as they are porous I guess).

I wish to build a speaker enclosure using them, as they are ready made
in various sizes, have an artistic appeal and if the sound reproduction
dose not suffer, It's a very attractive proposition!
These pots or "matkas" have walls which are usually 3-4 Milimeter
thick ( I can get them made thicker).

I did some tries by mounting 8 inch drivers on the "mouth", did some
stuffing inside using polyfill, but the observed sound reproduction did
not seem too "right" to me- I thought it was resonating too much (like
booming) at low freqs. and had a tinny feel to the sound..

Please have a look at the setup here:
http://www.geocities.com/madforeagles/DSC01343.JPG
http://www.geocities.com/madforeagles/DSC01344.JPG


My queries are, should I line the insides with some material to make it
air tight? how thick should I get them made? should I use the same
volume calculations for 'normal' enclosures for a driver? should I
stuff it with Polyfill?

Thanks!
Suraj

Yes spherical enclosers can work quite well because they do not have sharp
edges causing troubling acoustic diffraction patterns. These patterns can
color the sound by causing peaks and valleys in response and are partly
responsible for the "lousy" sound of some boxes. A spherical enclosure does
not automatically improve things, however, it has to be properly made of the
right volume and be properly damped without resonance.
The boomy sound you report was most likely was caused by too large a
volume and or air leaks causing a "porting effect." A second order system
requires a "sealed" box so be sure that all air leaks are sealed up and that
there are no holes or gaps anywhere in the sphere or speaker mounting. Yes,
you should fill the volume with fiber glass, dacron or some other material
which will damp resonances and increase the speaker loading.
The volume needed should be calculated: Vb = Vas/((Qtc/Qts)^2 - 1).
Where Vb is the required box volume (liters), Vas is a speaker parameter
(liters), Qts is the speaker Q and is a combination of the electrical and
mechanical Q (Qts = Qe*Qm/(Qe + Qm)). These parameters can be measured but
are usually supplied with a given speaker. Qtc is the required closed box Q
and should be around 0.7 for a second order system. The final closed box cut
off frequency will be: Fc = Qtc*Fs/Qts. Where Fs is the free air resonance
of the speaker.
If you follow these design guides and use a well made rigid clay sphere
of the right volume, Vb, you should wind up with a good speaker system. Of
course, this discussion is based on one speaker, the bass unit. Midrange and
tweeters have to be similarly handled.
Bob

 

[Ban]

That would be debateable. I suspect you may be remembering that
a spherical *source* is best for omnidirectional sound radiation,
but there is no easy way to get that with an enclosure. Some
folks have tried covering a spherical enclosure with many small
drivers. The problem there is that, as with any multi-driver system,
you have interactions between the driver coverage patterns.
A driver's output pattern is dependent upon frequency, especially
toward the top of its range. So if you try to position the drivers
so that their fields just meet for perfect coverage at one frequency
range, then at other ranges the fields will have overlaps or gaps.
That gives ragged frequency response due to phase cancellation, etc.



The pot "enclosure" you describe would be a classic Helmholtz
resonator... the exact opposite of what you want for sound
REproduction, though it might be a great musical instrument on
its own. The polyfill will help you somewhat, but I suspect that
the bigger problem is that the walls themselves are resonating
like a big bell. If you are determined to follow through with
this (for the sake of art, say) then you might want to try
coating the pot walls (inside or out) with something dense and
viscous, like pitch. As another post notes, these walls are pretty
thin to start with, which is making the problem worse.

Best regards,


Bob Masta

My speakers have sperical enclosures and sound quite phantastic, but the
spheres are made with 3 different layers and inside there is a lot of
dampening material and they are closed cabinets. See my page:
http://www.pupazzo.page.ms/